1. Field of the Invention
The present invention relates generally to the art of surface treatments for glass and more particularly to methods for lowering the surface energy of a glass article thereby rendering the surface non-wettable and lubricated.
2. The Prior Art
Ordinary glass comprises an acidic oxide such as silica and base metal oxides such as the oxides of alkali and alkaline earth metals. Ordinary soda-lime-silica glass is hydrophilic and absorbs moisture from the atmosphere. Since the base metal oxides are somewhat soluble in water, glass surfaces exposed to moisture are subjected to a dissolving action that tends to leach components out of the glass surface. Glass which is exposed to humid atmospheric conditions therefore has a tendency to discolor and develop a stain on its surface that reduces its transparency. The probable reactions of the glass surface are described as follows.
In the initial phase of weathering, a siliceous film of irregular thickness develops on the glass surface. The refractive index of the film differs from that of the underlying glass sheet because the film has a higher concentration of the alkali and alkaline earth metals. The result is discoloration which resembles a light film of oil in that it is multicolored, containing various shades of yellow, red, green and purple, referred to as iridescence.
As the weathering process continues, water absorbed by the hydrophilic glass surface leaches out sodium and calcium ions resulting in a solution of sodium and calcium hydroxides, a caustic solution capable of dissolving silicates present in the glass surface. The silicates form a powdery white scum on the glass surface producing a frosted appearance. The scum also contains sodium and calcium carbonates, presumably from the reaction of atmospheric carbon dioxide with the sodium and calcium ions leached out of the glass surface. While this silicate/carbonate scum may be washed off, a white, spotted discoloration and etching, referred to as stain, remains on the glass surface and can be removed only by grinding and polishing the glass surface.
The prior art discloses that iridescence and staining of glass sheets subjected to humid atmospheric conditions can be inhibited by contacting the glass surface with a slightly acidic agent having a pH between 4 and 7. The acidic agents, metallic salts of strong acids and weak metallic bases, may be sprayed in a solution onto the glass surface. Acidic gases such as SO.sub.2 and SO.sub.3 are also used to inhibit weathering effects on glass by treating the glass for about 30 minutes at temperatures of 300.degree. to 600.degree. C. in an atmosphere comprising 3 to 5 percent acidic gas. It is theorized that the acidic agents protect glass from weathering effects by neutralizing the alkali concentrated in the surface of the glass resulting in a neutral surface which does not dissolve to form a caustic solution as the glass surface absorbs moisture.
In U.S. Pat. No. 3,508,956, Moser et al note that the aforementioned acid treatment results in a light but permanent etch that decreases the initial transparency of the glass. Moser et al disclose a noncorrosive treatment whereby a glass surface is made resistant to iridescence, scum and stain by a residue of a basic solution of a metal ammino halide or a metal ammino nitrate on the glass surface. The method comprises contacting a glass surface at about 200.degree. C. with a solution of ammonium hydroxide and an amphoteric metallic ammino compound which dries to form a protective residue.
In U.S. Pat. No. 3,420,693, Scholes et al disclose a method for treating glass by forming a dual protective coating on a virgin glass surface. First, a hot glass surface is exposed to a heat decomposable tin compound to form a tin oxide coating on the glass surface. Second, a lubricious organic compound is coated over the tin oxide coating. The dual protective coating improves the durability of glassware such as bottles and jars.